This invention relates generally to communications and, more particularly, to high-speed data communications systems.
Plain Old Telephone Service (POTS) is typically deployed to individual subscribers over a twisted pair of wire. Today, in addition to voice services, more and more subscribers want high-speed data access to, e.g., the Internet, over this twisted pair. One technology that increases the transmission capacity over a twisted pair is Asymmetric Digital Subscriber Loop (ADSL). One version of ADSL increases the bandwidth of the twisted pair up to 1.1 Mhz (megahertz), which provides transmission capabilities up to 9 Mbps (millions of bits per second).
ADSL allocates different amounts of bandwidth between upstream communications and downstream communications (hence the term xe2x80x9casymmetricxe2x80x9d), with upstream communications having less bandwidth than downstream communications. In this context, there are different strategies for specific bandwidth allocation and different modulation methods available. For example, in the upstream direction, i.e., from a subscriber""s consumer premises equipment (CPE) to a central office (CO) (or local exchange carrier (LEC)) the upstream channel may have an allocated bandwidth from 25 Khz (kilohertz) to 138 Khz; while in the downstream direction, i.e., from the CO to the CPE, the downstream channel may have an allocated bandwidth from 138 Khz to 1.1Mhz. (The POTS voice channel (0 to 4 Khz) is unaffected by ADSL.). In this example, the upstream channel and downstream channel are disjoint and also adjacent. However, ADSL systems can be constructed where the upstream channel partially overlaps with the downstream channel. While this provides more bandwidth for the downstream signal, this also requires the use of echo cancellation techniques. Turning to modulation methods, carrierless amplitude phase (CAP) modulation or Discrete Multi-Tone (DMT) modulation can be used. (DMT is a form of orthogonal frequency division multiplexing (OFDM).)
One standard for ADSL transmission is ANSI T1.413. This standard specifies the use of DMT modulation, which utilizes multiple carriers (also sometimes referred to as subcarriers) for conveying information. In DMT modulation, the allocated frequency range is divided into K carrier channels, K greater than 1, each carrier channel separated by approximately 4 Khz. In such an approach, a DMT-based ADSL system transmits what is referred to as xe2x80x9cmulti-tone symbolsxe2x80x9d or xe2x80x9cDMT symbols.xe2x80x9d
Unfortunately, a DMT-based ADSL system is particularly susceptible to impulse noise (which, in the frequency domain goes across all frequencies). As a result, impulse noise hits can cause unrecoverable errors in a DMT-based ADSL system. In addition, the high-speed of these ADSL systems presents challenges in terms of providing equipment designs that not only provide protection against impulse noise but are also comparably lower in cost, i.e., have less processor complexity.
We have realized a method and apparatus for providing a coding technique in multi-carrier communications systems that not only provides additional protection against the above-mentioned impulse noise but also has comparably low cost, i.e., is less complex. In particular, and in accordance with the principles of the invention, a multi-carrier transmission signal is formed where each symbol value is transmitted in a redundant form over a plurality of consecutive symbol intervals. As a result, this symbol redundancy provides protection against impulse noise without requiring additional processor complexity.
In an embodiment of the invention, within a transmitter portion of ADSL DMT equipment an encoder operates on a DMT input signal representing f symbols/sec. to generate, for transmission, a DMT output signal representing Nf symbols/sec., where N greater than 1. In this example, the redundant form is illustrated by repeating each input symbol value in N consecutive symbol intervals in the DMT output signal.
In another embodiment of the invention a different type of redundant form is illustrated. Again, within a transmitter portion of ADSL DMT equipment an encoder operates on a DMT input signal representing f symbols/sec. to generate, for transmission, a DMT output signal representing Nf symbols/sec., where N greater than 1. In this example, every N DMT input symbol values are transmitted in 2N consecutive symbol intervals in the DMT output signal. These 2N consecutive symbol intervals in the DMT output signal include 1) N symbols whose values equal the respective N DMT input symbol values and 2) N symbols whose values are a function of the respective N DMT input symbol values.